1. Field of the Invention
The present invention relates to an optical pickup device, an information reproduction/recording apparatus, and an information processing apparatus.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 11-162003 discloses an information reproduction apparatus that reproduces information recorded on an optical disk. In this information reproduction apparatus, light is emitted on the optical disk, and the reflection light from the optical disk is divided and received by a plurality of light receiving members. The current signals obtained from the respective light receiving members are added by an adder, and the resultant signal is then converted into a voltage by a current/voltage converter (hereinafter referred to as “I/V converter”).
In accordance with the technique, an amplifier is mounted in the same package as the light receiving members. Since the I/V converter is also mounted on the optical pickup apparatus, a reproduction signal of the optical disk can be resistant to an adverse influence from noise, because it can be extended by 20 cm or more by a voltage signal that is relatively resistant to noise.
The adder that adds up the output signals of the respective light receiving members is also mounted on the optical pickup apparatus. In view of this, an adverse influence from noise can be avoided. In a case where the voltage signals converted from the output signals of the light receiving members are transmitted over a long distance through a plurality (four, for instance) of signal lines, with the noise superimposed on each signal line being N, the noise superimposed on the voltage signal after the adding operation by the adder is N×√{square root over (4)}=2×N. On the other hand, after the voltage signals are added up in the optical pickup apparatus, the resultant voltage signal is outputted to the circuit substrate through a signal line, so that the superimposed noise can be equivalent to N.
In the method disclosed in Japanese Laid-Open Patent Application No. 11-162003, however, there are problems with the dynamic range of the circuit. More specifically, since the RF signal is an addition signal of the output signals of the light receiving members, the RF signal has a signal level that is several times (four times in a case of a general optical system, because the four light receiving members are divided from one light receiving element) higher than the output of each of the light receiving members. Accordingly, even if the output range of the RF signal is set in the entire dynamic range, the range of the output signal of each of the light receiving members is limited to one fourth of the dynamic range. Since the output of each of the light receiving members is normally used for generating a servo signal for controlling the operation of the pickup device, it has a low bandwidth, and is relatively resistant to noise. However, with a larger number of light receiving members or a higher reading rate of the optical disk, the servo signal might be adversely influenced by noise.
When information is to be recorded on a DVD-R disk, for instance, the quantity of light for forming recording marks equivalent to “1” is more than 10 times as large as the quantity of light for recording spaces equivalent to “0”. To prevent saturation of the output of the RF signal for the large quantity of light for forming the recording marks, the dynamic range of the output of each of the light receiving members during the space recording operation is only 1/40 of the RF signal. As a result, the output signal of each of the light receiving members is limited to an extremely narrow range.
Furthermore, there is another problem with the prior art. To restrict the noise on a signal line that serves as a transmission path, a signal that has the opposite polarity to the RF signal is transmitted at the same time as the RF signal (the differential output), and the difference is removed at the receiving end, so as to remove the noise in the in-phase component superimposed on both signals. Throughout this specification, the complementary signal for the RF signal is referred to as “RF−”. In a general differential output operation, if the RF signal is a signal on the positive side with respect to a predetermined reference voltage Vref, the RF− is a signal obtained by inverting the RF signal about the reference voltage Vref. As a result, the RF− has the opposite polarity to the RF signal. Alternatively, when the outputs of the respective light receiving members are added up, the outputs of the light receiving members are inverted at the same time. In this example, the output voltage of RF− is situated on the negative side with respect to the reference voltage Vref. As a result, there is a problem that a twice as wide dynamic range is required.